Enhanced oil recovery processes, which are utilized to increase the amount of hydrocarbon production from a subterranean reservoir, are becoming common practice within the petroleum industry. One of the most frequently utilized enhanced oil recovery processes includes injecting a gas into the subterranean reservoir to displace the oil. Oil displacement is primarily achieved through mechanisms including oil swelling and viscosity reduction. For example, the injected gases are typically miscible with the lighter components of the crude oil such that as they mix, the composition or phase behavior of the crude oil is altered, thus improving the flowability of the oil. Application of such gas flooding techniques, however, has historically been limited due to the accessibility of nearby gas sources. For example, the gas to be injected into the reservoir typically needs to be transported from a production source. This may not prove to be economically feasible as sufficient gas sources are typically not adjacent to such reservoirs, especially ones which are substantially pure and available for direct use in an oil field.
Carbon dioxide is one of the gases predominantly employed for enhanced oil recovery gas flooding processes. Sufficient sources of carbon dioxide needed for such commercial exploitation typically include carbon dioxide producing facilities, fossil fuel combustion, and natural underground deposits. However, the costs associated with building a dedicated carbon dioxide producing facility at each oil field or constructing a high-pressure pipeline for transporting pure carbon dioxide to the reservoir field are often prohibitive. Additionally, carbon dioxide flooding processes have not proven to be beneficial in subterranean reservoirs containing heavy or extra heavy oils, as the gas typically does not develop any significant miscibility due to the lighter components of the crude oil not being present.
Subterranean reservoirs containing heavy or extra heavy oils, which generally have an API gravity of less than about 20 degrees API, therefore, often utilize a thermal recovery process to increase the amount of hydrocarbon production from the reservoir. By introducing heat into the reservoir, such as through steam injection or in-situ combustion, the viscosity of the oil is reduced sufficiently to allow the oil to flow towards producing wells. However, as previously described, such steam generation and combustion processes naturally produce carbon dioxide that can be captured to prevent its released into the atmosphere. Since it has not proven beneficial in heavy oil reservoirs to utilize the captured carbon dioxide in gas flooding processes, the carbon dioxide is typically transported elsewhere in a high pressure pipeline. For example, the carbon dioxide can be shipped to a carbon dioxide consumer, an underground storage facility, or a reservoir utilizing a gas flooding process. In some instances, depleted reservoirs can be utilized for carbon sequestration, which serves to mitigate the accumulation of greenhouse gases in the atmosphere.
While such carbon capture and storage techniques mitigate the potential impact on the environment, the costs associated with transporting the carbon dioxide can be prohibitive. In addition, once the heavy oil is produced from the reservoir, it still must undergo upgrading prior to shipment. Accordingly, diluents such as naphtha or synthetic crude oil are typically added to the heavy oil to reduce its viscosity such that it can be pumped with less difficulty.
It has been proposed to transport mixtures of crude oil and normally gaseous carbon dioxide such that the carbon dioxide acts as a diluent reducing the viscosity and pour point of the oil while being flowed through a pipeline. After transport, the carbon dioxide can then be separated from the crude oil. For example, U.S. Pat. No. 3,596,437 titled, “Use Of Carbon Dioxide In A Crude Oil Pipeline” discloses a method of transporting crude oil in a pipeline by mixing the crude oil with a fluid containing at least fifty percent by volume of carbon dioxide and less than ten percent by volume of ethane. As described in the specification of this patent, “At pipeline conditions, the fluid rich in carbon dioxide is a liquid and sufficiently soluble in the crude oil to accomplish a reduction in viscosity and pour point of the crude oil.” See Column 1, Lines 61-63. Disclosed pipeline conditions include operating temperatures ranging from less than about −5 degrees Fahrenheit to about 70 degrees Fahrenheit and pipeline pressures below 500 p.s.i. (See Column 2, Line 73-Column3, Line 37).
As will be disclosed herein, Applicants propose a method for transporting a mixture of carbon dioxide and heavy oil in a pipeline under significantly different conditions.